1. Field of the Invention
The present invention relates to novel silanol-containing triarylamine derivatives useful as a hole transporting material.
2. Description of the Related Art
Triarylamine derivatives have been used as a photoconductive material of electrophotographic photoreceptors. For example, U.S. Pat. No. 3,180,730 discloses a photoreceptor using a triphenylamine compound as a dye sensitizer. Japanese Patent Laid-Open No. 65440/1983 discloses a photoreceptor using a triarylamine derivative as a charge transporting material.
The triarylamine derivative disclosed in Japanese Patent Publication No. 110940/1995 is excellent as a hole transporting material and has been used for organic light emitting diode or organic thin-film solar cells. In the production of such devices by using a wet process, a solution obtained by dissolving the hole transporting material, together with an appropriate binder resin, in an organic solvent and the resulting solution is applied. As the binder resin, thermoplastic resins such as polycarbonate resin, polyester resin, acrylic resin, and polystyrene resin and thermosetting resins such as polyurethane resin and epoxy resin are considered.
Japanese Patent Laid-Open No. 346356/1992, on the other hand, discloses a method of using a resin obtained by dispersing a thermosetting polysiloxane resin in a thermoplastic resin as a component substance of a charge transporting material. Polysiloxane resins have characteristics which other resins do not have such as transparency, anti breakdown, light stability, and low surface tension. However, due to lack of compatibility with organic compounds, polysiloxane resins cannot be used singly as a component resin of a charge transporting material.
In order to solve these problems, Japanese Patent Nos. 3614222 and 4392869 disclose compounds having, in the triarylamine structure thereof, an alkoxysilyl group. These compounds are excellent in compatibility with a silicon-based resin such as polysiloxane and can provide a uniform organic thin film free of crystal precipitation or pin holes.
When an organic light emitting device is fabricated, a hole transporting material is usually stacked on an inorganic material. Due to the hydrolysis of an alkoxysilyl group, a silicon-oxygen bond is formed on the surface of the inorganic material, which is presumed to improve the transferring efficiency of charges on the interface.
The alkoxysilyl group disclosed in Japanese Patent Nos. 3614222 and 4392869 generate VOC (volatile organic component) upon hydrolysis, which causes the burden on the environment. In addition, for the hydrolysis of an alkoxysilane, a catalyst for accelerating the reaction is usually employed. The catalyst also serves to condense silanol groups and the reaction mixture inevitably becomes a mixture with a siloxane compound, which may possibly deteriorate the performance. Furthermore, when these alkoxysilanes are used as a hole transporting material, they are required to be purified highly. It is however difficult to purify them through distillation because they have a high boiling point. Using column chromatography for purification, on the other hand, may cause another problem that alkoxysilanes are easily hydrolyzed and adsorbed in the column.
There is therefore an eager demand for the development of a triarylamine derivative having more suitable properties as a hole transporting material.